1. Field of the Invention
The present invention relates to an optical low pass filter for use in an imaging device such as, a solid state imaging device or a single tube type color imaging device, for alleviating spurious mono color signals and/or spurious color signals.
2. Description of the Prior Art
It is well known that a solid state imaging device and a single tube type color imaging device employ an array of color filters to give color signals. If radiations from the scene to be imaged or photographed such as, for example, a person, or a group of persons and/or a landscape, contains a component of frequency substantially equal to the pitch of each neighboring filter of the filter array, the subsequent detection of this frequency component tends to result in a generation of a spurious color signal.
It is also well known that a single flat panel type color imaging device employs a solid state image sensor with their picture elements or sensing cells arranged discretely and regularly and, therefore, if radiations from the scene to be imaged or photographed contains a high frequency component of a periodicity smaller than the pitch (periodicity) of each neighboring picture elements of the solid state image sensor, the subsequent detection of this high frequency component tends to result in a generation of a spurious signal due to recurrent.
In view of a foregoing, the solid state imaging device or a single tube type color imaging device generally employs an optical low pass filter with the aim of substantially limiting the high frequency component contained in the radiations from the scene to be imaged or photographed, thereby alleviating the generation of the spurious color signal or the spurious mono color signal, respectively.
FIG. 1 of the accompanying drawings illustrate, in schematic longitudinal representation, a prior art solid state imaging device (See the Japanese Laid-open Patent Publication No. 61-149923, published July 8, 1986). The device of FIG. 1 is a solid state image sensor, which comprises a casing 7, which opens towards a pick-up lens or photo-taking lens 2, and an array of color filters, generally identified by 3 and housed within the casing 7 so as to face towards the opening of the casing 7 that is closed by a transparent protective glass 4. Positioned between the photo-taking lens 2 and the solid state image sensor 1 is an optical low pass filter 6. The prior art optical low pass filter 6 shown therein is generally employed in the form of a birefringent plate such as made of quartz and is, therefore, not only expensive to make, but also incapable of being manufactured on a mass production basis.
Accordingly, attempts have been made to provide an optical low pass filter wherein, in place of the birefringent plate, an inexpensive and mass-produceable diffraction grating is employed. (See the Japanese examined Patent Publication No. 61-38452, published Aug. 29, 1986, which corresponds to the U.S. patent application No. 28,368, filed Apr. 9, 1979 now U.S. Pat. No. 4,255,109).
In the prior art color imaging device employing the diffraction grating, in place of the birefringent plate, for the low pass filter 6, it has been found that, although a low frequency bandpass characteristic exhibited thereby is satisfactory, there is a problem in that, when a diaphragm employed in the color imaging system is stopped down, an image of the diffraction grating tends to appear within the frame of a picture being taken of the scene. This phenomenon will now be discussed in detail.
Referring to FIG. 2, there is shown a graph illustrating the modulation transfer function (MTF) characteristics of the diffraction grating of a type having a generally rectangular wavy cross-section (a generally battlement-like cross-section), wherein fc1 and fc2 represent respective cut-off frequencies. Each of these cut-off frequencies fc1 and fc2 can be expressed by the following respective equation. EQU fc1=a/b.lambda. (1) EQU fc2=(d-a)/b.lambda. (2)
wherein d represents the periodicity of the diffraction grating, a represents the width of each of elongated parallel projections of the diffraction grating, b represents the distance between the diffraction grating and the image plane of the imaging system or the front surface of the solid state image sensor where an image of the scene is formed by the photo-taking lens, and .lambda. represents the wavelength of the incident light radiated from the scene to be photographed.
Accordingly, once the distance b between the diffraction grating and the image plane of the imaging system is determined, optimization of the periodicity d and the width a of each elongated projection of the diffraction grating is effective to alleviate the generation of the spurious color signal and/or the spurious mono color signal.
The prior art color imaging device wherein the optical low pass filter is employed in the form of the diffraction grating is illustrated in FIG. 3 in schematic longitudinal representation. The diffraction grating identified by 5 is disposed so as to intervene between the photo-taking lens 2 and the transparent protective glass plate 4 secured to the casing for the solid state image sensor 1 and, therefore, the distance b between the diffraction grating 5 and the image plane 8 of the imaging system necessarily tends to be relatively long, for example, 1 mm or more, and, accordingly, the periodicity of the diffraction grating 5 determined by the equations (1) and (2) above generally amounts to a value three times the periodicity of the picture elements of the solid state image sensor 1. This in turn results in that the periodicity of the image of the diffraction grating 5 projected by the photo-taking lens 2 onto the solid state image sensor 1 (that is, the effective periodicity of the diffraction grating 5) tends to be large enough to assume a value three times or more than the periodicity of the picture elements of the solid state image sensor 1.
An example of an output signal outputted from the solid state image sensor 1 when the effective periodicity of the diffraction grating 5 is three times the periodicity of the picture elements is shown in FIG. 4. As shown therein, since the output signal from the solid state image sensor 1 is modulated in strength by the diffraction grating 5, an image of the diffraction grating 5 appears in the form of a pattern of alternating dark and bright irregularities within the frame of the picture being taken of the scene. A similar phenomenon occurs even where the effective periodicity of the diffraction grating employed in the solid state imaging device is of a value greater than 3 times the periodicity of the picture elements by the same reasoning.
Even in the single flat panel type color imaging device referred to hereinbefore, the distance between the diffraction grating and the image plane of the imaging system is generally 1 mm or more. Therefore, when the periodicity d of the diffraction grating 5 is calculated on the basis of the previously described equations (1) and (2) to determine the appropriate cut-off frequencies, the result shows that the periodicity d tends to be of a value equal to or greater than three times the periodicity of the picture elements of the solid state image sensor. This will be hereinafter discussed with reference to the prior art single flat panel type color imaging device wherein an RGB filter array made up of a plurality of triads of red (R), green (G) and blue (B) stripe filter elements is employed as a color filter array.
FIG. 5 illustrates R, G and B output signals generated from the single flat panel type color imaging device when the effective periodicity of the diffraction grating is four times the periodicity of the picture elements. The luminance signal corresponding to each triad of different color filter elements of the color filter array is modulated in strength and, therefore, a pattern of alternating dark and bright irregularities appears within the frame of the picture being taken of the scene, which irregularities represent an image of the diffraction grating.